command = ""
started = False
while True:
    command = input("> ").lower()
    if command == "start":
        if started:
            print("Car is already started.")
        else:
            started = True#重新设定布尔值
            print("Car started.")
    elif command == "help":
        print("""
start -- start
stop -- stop
quit -- quit """)
    elif command =="stop":
        if not started:
            print("Car is already stoped.")
        else:
            started = False
            print("Car stoped.")
    elif command == "quit":
        break#停止
    else:
        print("Can not understand.")
